1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and, particularly, to a heat-sensitive recording material which is superior in the reproduction of gradation and enables recording at high density.
2. Description of the Related Art
The heat-sensitive recording method has the following advantages: (1) developing is unnecessary; (2) when the substrate is paper, paper quality is close to that of general paper; (3) the handling thereof is easy; (4) developed color density is high; (5) the recording device is simple and inexpensive; and (6) this method is free from noises when recording. Therefore, the application of the heat-sensitive recording has spread to the fields of facsimiles and printers and the fields of labels such as POS.
With this background, transparent heat-sensitive recording materials enabling direct recording using a thermal head have been recently developed for the purposes of coping with a trend toward multi-coloration and projecting an image by using an overhead projector (OHP) and observing an image directly on a light table, and demands with respect to the quality of an image are increasing on one hand. On the other hand, a rise in color development against printing energy tends to be acute as a progress in the high-sensitization of heat-sensitive recording materials is made corresponding to high speed recording, an impression that the gradation of an image has been sacrificed.
In view of this, in order to attain the compatibility of sensitivity with gradation, a heat-sensitive recording material has been developed in which a second color developing layer and a first color developing layer having the same developed color tone as the second color developing layer and laminated on the second color developing layer are laminated on a substrate and the first color developing layer is made to develop a color at a lower temperature than the second color developing layer (Japanese Patent Application Publication (JP-B) No. 6-30953). This invention gives priority to the color development of the first layer in low temperature printing and makes use of color development of the second color developing layer in high temperature printing, however, it possesses the problem that only insufficient image density can be obtained.
Another heat-sensitive recording material has been proposed which contains a water-soluble resin (polyvinyl alcohol) and a solubilizing auxiliary and comprises two heat-sensitive layers including two kinds of capsules (see, e.g., Japanese Patent Application Laid-Open (JP-A) No. 2000-280619). However, since the two kinds of capsules have the same average particle diameter (0.70 μm) in this material, improvement of both color density and gradation reproducibility is insufficient. Still another heat-sensitive recording material has been proposed which contains stearic acid amide in a protective layer and comprises two heat-sensitive layers including two kinds of capsules (see, e.g., JP-A No. 2002-67500). However, since the two kinds of capsules have the same average particle diameter (0.35 μm) in this material, improvement of both color density and gradation reproducibility is still insufficient.